Compare & Contrast

Python has several pairs of features that look similar but behave differently. Using the wrong one leads to subtle bugs or confusing errors. This section explains the most important distinctions in plain English, with examples you can run yourself.

== vs is — == compares values; is compares object identity. For None and for small integers, the difference matters: you should write if x is None and avoid if x == None. We explain when to use which and why you might see unexpected results with lists or strings. Related concept: mutable vs immutable and how identity affects copies.

.copy() vs deepcopy() — A shallow copy (e.g. list.copy() or dict.copy()) creates a new container but does not copy the objects inside it. If the list contains other lists, changing a nested list affects both the original and the copy. copy.deepcopy() recursively copies everything, so the two structures are fully independent. We show when you need deep copy and when shallow copy is enough, and how this ties into data types and mutability.

range vs xrange — In Python 2, xrange was a memory‑efficient way to iterate over indices; range built a full list. In Python 3, range behaves like the old xrange (it is a lazy sequence), and xrange no longer exists. We summarise what changed and how to think about range in modern Python, including in loops with an index.

After reading these, you may want to go deeper in Core Concepts (e.g. lists vs tuples, shallow vs deep copy) or in the Errors section when a comparison clarifies an error message.